Water vapour sensors, in particular humidity sensors, also known as hygrometers, are well known. Hygrometers are used for many purposes: meteorology; monitoring environments within buildings; controlling air conditioning; in industry; making paper; horticulture; monitoring foodstuffs in supply chains; and many other uses.
Humidity is difficult to measure. Many techniques for measuring humidity are based on relatively large physical sensors. Small electronic sensors are used but such sensors are based on semiconductor techniques: and require semiconductor production facilities. For example it is known to combine carbon nanotubes with field effect transistors for sensing humidity.